Bayard-Alpert (BA) ionization gauges are the most common non-magnetic means of measuring very low pressures and have been widely used worldwide since being disclosed in 1952 in U.S. Pat. No. 2,605,431, which is herein incorporated by reference in its entirety.
A typical ionization gauge includes an electron source, an anode, and an ion collector electrode. For the BA gauge, the electron source is located radially outside of an ionization space (anode volume) defined by the anode. The ion collector electrode is disposed within the anode volume. Electrons travel from the electron source toward and into the anode. In their travel, the electrons impact molecules and atoms of gas, constituting the atmosphere whose pressure is to be measured, and create ions.
The ions are attracted to the ion collector electrode by the electric field inside the anode. The pressure of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S) (Iion/Ielectron), where S is a coefficient with the units of 1/Torr and is characteristic of a particular gauge geometry, and electrical parameters and pressure range. One well known problem in the art is that the hot cathode filament may react with the gaseous substances to be measured.
Ionization gauges with cold cathode electron emitters are also known, such as, for example, U.S. Pat. No. 5,278,510 to Baptist et al. (hereinafter “Baptist”), which is also incorporated by reference in its entirety. Baptist discloses an ionization gauge that includes a cold cathode instead of the hot cathode filament. Baptist's ionization gauge includes a number of cathode electrodes, which provide a bias voltage for a number of tipped micro-points (Spindt emitters). Electrons are emitted from the micro-points through field emission to ionize the gas, and thus measure pressure once the ions are collected, similar to the BA ionization gauge. However, after a short period of time, the tipped micro-points, which are bombarded by ionized gaseous material will erode. This degrades the emissivity, and leads to termination of electrons from the micro-tips. This renders the ionization gauge unable to measure pressure and causes the gauge to fail.